The present invention relates to optical dispersion compensator, and in particular to a miniature optical dispersion compensator with low insertion loss by combining a dual-fiber collimator and an etalon into a single small device.
Rapid recent progress in the development of erbium doped fiber amplifiers (EDFA) has made possible ultra-long-distance optical transmission without the use of regenerative repeaters. Chromatic dispersion of single-mode optical fiber has becomes a bottleneck for high-speed long-haul optical transmission systems. It is known that selected optical fibers with opposite dispersion can be used for equalizing group-delay incurred by optical signals traveling over a long distance. Dispersion-compensating fiber (DCF) are designed such that the wave-guide dispersion is enhanced by modifying the fiber refractive profile so that it becomes greater than the material dispersion. Based on this method, very long length of dispersion-compensating fiber will be required to compensate the dispersion of an even modest length of a transmission fiber. Also, an optical amplifier will be needed to compensate for the insertion loss of the dispersion-compensating fiber (DCF), which is generally higher than that of conventional single mode fiber.
Other methods include the use of chirped fiber Bragg gratings, and the use of planar light-wave circuits. Fiber Bragg gratings are generally not practical for field applications due to their relatively narrow bandwidth, and relatively fixed wavelength. Planar wave-guide circuits (PLC) are also of relatively narrow bandwidth, and relatively difficult to be fabricated. MEMS technology has attracted many attentions in tunable dispersion compensating filter. Although MEMS (Micro Electro Mechanical System) based dispersion compensating device can be designed and fabricated with tunable function, they are not quite mature and cost-effective in the current stage.
U.S. Pat. No. 5,023,947 discloses an optical equalization receiver comprising a reflective Fabry-Perot etalon and a piezoelectric transducer for dynamically controlling the optical path length of the etalon. The reflected signal from the Fabry-Perot etalon with a rear reflective mirror of 100% reflectivity has a Lorentzian group delay response with optical frequency. By tuning the etalon such that its dispersion has appropriate sign and magnitude at the optical signal wavelength, dispersion compensation can be achieved. Since this is a reflective device, a 3 dB coupler or an optical circulator would be required to separate input and output signals.
U.S. Pat. No. 5,557,468 discloses a Similar two-port Fabry-Perot etalon bsaed design for dispersion compensation. The difference between this design and the above reflective based Fabry-Perot resonator design is that the rear mirror""s reflectivity is not exact 100%. It is a little bit lower than 100% for the output port monitoring. The advantages of this design are two folds: (1) it provides a device with a monitoring port on the transmitted side of the etalon, and the amplitude response of the monitor port provides a much higher signal contrast ratio; and (2) The local control scheme allows the compensation device to be located virtually anywhere in the system, even at the transmitter end.
But all these two designs need a 3 db coupler or an optical circulator to separate the input signal and the compensated signal. That will increase the insertion loss, and at the same time increase the cost of the device or system. In order to tune the etalon cavity to locate the ITU (International Telecommunication Union) wavelength position, piezo-electric and MEMS actuator have been used to adjust the cavity length, which further increases the complexity and cost of the optical system.
For foregoing reasons, it would be an advance in the art to provide an optical dispersion compensator that eliminates the use of a 3 dB coupler or an optical circulator. It is an especially welcome advance to provide a miniature optical dispersion compensator with low insertion loss by combining a dual fiber collimator and an etalon into a single small device.
It is a primary object of the present invention to provide an optical dispersion compensator for a predetermined range of wavelengths with relatively small size and low insertion loss. The compensator has a Fabry-Perot etalon and a dual-fiber collimator having a dual-fiber pigtail and a GRIN lens. At least one negative dispersion region can be obtained in the output response of the optical dispersion compensator and dispersion compensation can be achieved. The insertion loss of the optical dispersion compensator can be as low as 0.25 dB.
It is a further object of present invention to provide an optical dispersion compensation system for a predetermined range of wavelengths comprising a plurality of optical dispersion compensators with same or different optical parameters to obtain at least one negative dispersion region. By cascading two or more optical dispersion compensators together, it is possible to extend the range of the output response considerably with respect to both time delay and operating wavelengths. A two-stage optical dispersion compensation system can reach an insertion loss as low as 0.5 dB.
It is yet another object of the present invention to provide a method of fabricating an optical dispersion compensator for a predetermined range of wavelengths with low insertion loss to obtain at least one negative dispersion region. The phase shift of the optical dispersion compensator can be adjusted and the middle wavelength of the predetermined range of wavelength can be located at the middle of the negative dispersion region.
These and numerous other objects and advantages of the present invention will become apparent upon reading the detailed description.
In accordance with the present invention, an optical dispersion compensator for a predetermined range of wavelengths has a Fabry-Perot etalon and a dual-fiber collimator is provided. The Fabry-Perot etalon has a front reflective mirror and a rear reflective mirror in a parallel spaced relationship to form a cavity between the reflective mirrors. The reflectivity of the front reflective mirror is substantially smaller than the reflectivity of the rear reflective mirror. The reflectivity of the rear reflective mirror is substantially 100%, or between 97% and 100%. The dual-fiber collimator has a dual-fiber pigtail and a GRIN lens. The dual-fiber pigtail has an input fiber and an output fiber. An optical signal is coupled into the input fiber and then collimated by the GRIN lens into a collimated incident beam which is incident on the Fabry-Perot etalon with an incident angle. The collimated incident beam is reflected back by the Fabry-Perot etalon into a collimated reflected beam which is collected by the GRIN lens and then coupled into the output fiber.
By adjusting the separation between the input fiber and the output fiber of the dual-fiber pigtail, the collimated incident beam will change its incident angle and reflected angle. By tuning this angle, the group delay pattern of the output optical signal through the output fiber will shift along with wavelength and the middle wavelength of the predetermined range of wavelengths can be located in the downturn slope range of the group delay pattern (or in other words, in the negative dispersion region) therefore dispersion compensation can be achieved. Generally, the middle wavelength of the predetermined range of wavelengths can be any ITU wavelength corresponding to an ITU channel or other operating wavelengths in the art. By aligning the dual-fiber pigtail and the GRIN lens, the insertion loss of the optical dispersion compensator can reach as low as 0.25 dB.
In accordance with the present invention, the front reflective mirror and the rear reflective mirror of the optical dispersion compensator can be two side surfaces of a solid substrate coated with reflective films.
The optical dispersion compensator of the present invention further has a spacer made of highly temperature stable material, e.g. Zerodur glass. Under this situation, the end surface of the GRIN lens is used as the front reflective mirror, the rear reflective mirror can be a coated mirror. The etalon cavity is filled with air and the spacer is positioned between the front reflective mirror and the rear reflective mirror to assure the length of the cavity.
The optical dispersion compensator of the present invention further has a tube holding the GRIN lens. The tube has a same temperature coefficient as that of the GRIN lens. Similarly, the end surface of the GRIN lens is used as the front reflective mirror and the rear reflective mirror can be a coated mirror. The etalon cavity is filled with air and the spacer is positioned between the end surface of the tube and the rear reflective mirror to assure the length of the cavity.
In accordance with the present invention, there is further provided an optical dispersion compensation system for a predetermined range of wavelengths comprising a plurality of optical dispersion compensators with same or different optical parameters to obtain at least one negative dispersion region. By cascading two or more optical dispersion compensators of the present invention together, it is possible to extend the range of the output response considerably with respect to both time delay and operating wavelengths. In accordance with one aspect of the present invention, it is provided an optical dispersion compensation system by cascading together two optical dispersion compensators of the present invention with different optical parameters. The two-stage optical dispersion compensation system can reach an insertion loss as low as 0.5 dB.
In accordance with another aspect of the present invention, a method for fabricating an optical dispersion compensator for a predetermined range of wavelengths is provided, comprising coating the front surface and the rear surface of a etalon, fixating a GRIN lens with the etalon, choosing a dual-fiber pigtail having a input fiber and a output fiber with a predetermined separation between them, aligning the dual-fiber pigtail with the GRIN lens fixated with the etalon, and fixating the dual-fiber pigtail with the GRIN lens fixated with the etalon. At least one negative dispersion region can be achieved in the output response of the compensator.
The method of the present invention can further comprise a step of machining the etalon into an adaptable size to the diameter of the GRIN lens.
The step of fixating the GRIN lens with the etalon of the method of the present invention can further comprise applying an adhesive, e.g. UV glue or epoxy glue (e.g. 353 NDTQ). So can the step of fixating the dual-fiber pigtail with the GRIN lens fixated with the etalon.
The step of aligning the dual-fiber pigtail with the GRIN lens fixated with the etalon can further comprise coupling an light source into the input fiber and coupling the optical signal from the output fiber into a optical spectral analyzer, such that the phase shift of the optical dispersion compensator can be adjusted and the middle wavelength of the predetermined range of wavelengths can be located substantially in the middle of the negative dispersion region.
Advantageously, the present invention eliminates the use of a 3 dB coupler or an optical circulator in the prior arts, which not only simplifies and minimizes the overall system but also substantially lowers the cost. Also, the optical dispersion compensator of the present invention can achieve an insertion loss as low as 0.25 dB, which makes the application of multiple-stage optical dispersion compensation system possible and practical.
The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The figures and the detailed description below will more particularly exemplify these embodiments.